Antimetabolites

Antimetabolite drugs were among the first effective chemotherapeutic agents discovered. Classified as folic acid, pyrimidine or purine analogues, these compounds have similar chemical structures to molecules the body uses in nucleic acid (DNA and RNA) synthesis. Antimetabolites are similar to chemicals needed for normal biochemical activity, but differ enough that they interfere with normal cell function. Generally, antimetabolites induce cell death during the S phase of cell growth when incorporated into RNA and DNA or inhibit enzymes needed for nucleic acid production. These agents are used for a variety of cancer therapies, including leukemia, breast, pancreatic, ovarian, and gastro-intestinal cancers.

Antimetabolites is a broad term, and could potentially refer to any drug that interferes with metabolic pathways, by inhibiting enzymatic reactions. Why can’t we just make more chemical compounds that get in the pathway? The problem for drug designers is that metabolic pathways are hard to figure out, and making compounds to interfere with them without causing other problems is fiendishly difficult.

Antimetabolite medicines used to fight cancer (e.g. methotrexate) slow the synthesis of pyrimidine and purine that cells in the S-Phase use to build new DNA molecules. (Methotrexate is an analog for folic acid.) They are characterized by low molecular weights.

Both the pyrimidine bases (uracil, cytosine), and the purine bases (adenine, guanine) are building blocks in the synthesis of DNA and RNA nucleotides. In the replication process, nucleotides combine to form DNA strands. It is less clear how the purine antagonists function, but they may inhibit normal production of DNA.

Pyrimidine Compounds

Duschinsky synthesized and Heidelberger, in 1957, introduced 5-flurouracil (5-FU). 5-FU is a pyrimidine base containing a fluoride atom at the 5 carbon position on the ring. Uracil is a naturally occurring pyrimidine base used in nucleic acid synthesis. It is converted to thymidine by enzyme action. 5-FU is similar in structure to uracil and is converted to two active metabolites (FdUMP and FUTP) that inhibit the activity of the enzyme thymidylate synthetase. The enzyme normally converts uracil to thymidine by adding a methyl group at the fifth carbon of the pyrimidine ring. 5-FU mimics the natural base and functions to inhibit DNA synthesis. The carbon group cannot be added because of the fluoride atom at the five position. Normal DNA synthesis fails. dUTP and FdUTP are incorporated into DNA so that it cannot function normally. In addition, FUTP is incorporated into RNA leading to faulty translation of the RNA. Thus, the synthesis of multiple forms of RNA (messenger, ribosomal, transfer and small nuclear RNAs) is blocked. These combined actions on DNA and RNA are cytotoxic to the rapidly dividing cancer cells.

5-FU is used for the treatment of many malignancies: breast, head and neck, adrenal, pancreatic, gastric, colon, rectal, esophageal, liver, and G-U (bladder, penile, vulva, prostate) . 5-FU may be administered by bolus IV infusion or continuous IV infusion over two days every 2-3 weeks or by oral ingestion. In addition, it may be used to treat skin cancers (basal cell and keratosis) by topical application.

Other pyrimidine antagonists include: cytarabine, capecitabine, gemcitabine and decitabine. Cytarabine (aka arabinosylcytosine) is a deoxycytidine base compound that is converted to its active metabolite, ara-CTP. This base is incorporated into DNA and causes strand termination. The cancer cell is unable to divide. This drug has proven effective in acute non-lymphocytic, lymphocytic, myelogenous , and chronic myelocytic leukemias, as well as leptomeningeal carcinomatosis and non-Hodgkin’s lymphoma. Capecitabine is an oral 5-FU pro-drug. It is converted to 5-FU by liver and tumor cells. It is used as adjuvant therapy in colon and breast metastasis. Gemcitabine is an ara-C pro drug which is activated by intracellular phosphorylation. This inhibits DNA and RNA synthesis. It is a first line treatment of pancreatic, metastatic breast, bladder, ovarian and non-small cell lung cancers. Finally, decitabine is phosphorylated and directly incorporated into DNA. Once part of the cell’s DNA, it stops methylation by inhibiting DNA methyltransferase and thereby induces cell death. It may also restore normal gene function controlling cell proliferation. It is used therapeutically in myelodysplastic syndrome.

Purine Compounds

There are few clinically useful purine antagonists. Scientists conjecture that these purine antagonists stop synthesis by decreasing the production of the purine bases or that the antagonist molecules themselves may be incorporated into the DNA strands during synthesis and halt cell replication. Without adequate amounts of the purine bases, nucleotide production stops and the cancer cell dies. Fludarabine or 2-fluoro-ara-amp is an antimetabolite of the purine class. It functions as a pro-drug. It is dephosphorylated to F-ara-ATP and enters the cancer cell. Upon incorporation into the DNA strand, it halts strand lengthening. The drug has been employed to treat refractory chronic lymphocytic and chronic B cell leukemias, non-Hodgkin’s lymphoma, and T- cell lymphoma. 6-Mercaptopurine (6-MP) is another purine agent used against acute lymphocytic leukemia. It is active in the S phase of cell proliferation. When it is incorporated into DNA and RNA, the nucleic acids are rendered useless. 6-MP may also act through inhibition of de novo synthesis of the purine bases. Genetic mutation may lead to purine resistance.

Adenosine deaminase inhibitors are a class of compounds that can be classified under the purine antagonist unbrella. These include Cladribine and Pentostatin.

Folate Antagonists

Folic acid is a necessary compound for the production of nucleotides. It was empirically observed in patients with leukemia that diets low in folate produced lower white cell counts than observed in leukemic patients on normal folate diets. In 1948, a folate antagonist was found effective in childhood leukemia. The antifolate medication methotrexate became an early chemotherapy drug. Intracellulary (between cells) folate is converted by the enzyme dihydrofolate reductase (DHFR) to dihydrofolate. This compound is then reduced to tetrahydrofolate (THDF). THDF acts as a carbon carrier compound that donates methyl groups to end target molecules through the enzymatic action of thymidine synthetase. DHFR is continuously used in this process and is the site where the folate antagonists function. The drugs impede enzyme action and hence interfere with nucleotide formation.

Methotrexate binds to DHFR reversibly and inactivates it. This prevents methylation and decreases available supplies of purine and thymidine bases for new DNA and RNA synthesis. Methotrexate is active in the S phase of cell growth. It is effective in many malignancies. Breast, head and neck, colorectal, non-Hodgkin’s lymphomas, osteosarcoma, bladder and choriocarcinoma are treated with methotrexate. It is also used in acute lymphocytic leukemia, and some types of meningeal carcinomas. Methotrexate is the most common folate antagonist used today even though others have been developed.

Drug resistance is a primary complication of treatment with methotrexate. Decreased drug transport into the cell can occur. A lower and less effective dose of methotrexate is observed intracellulary. Genetic mutations and alterations in gene activity may occur as well which alter binding constants to the enzymes or causes increases in the DHFR enzyme within the cell.

Pemetrexed is a folate antagonist used in the treatment of mesothelioma and non-small cell lung cancer. Pemetrexed is combined with cisplatin (an agent which promotes DNA cross-linking) to treat those cancers. Pemetrexed acts like methotrexate. It hinders multiple enzymes needed for de novo production of the thymidine and purine nucleotides. Normal DNA and RNA production is prevented. Pralatrexate is also approved for use against cancer, but it is less often used.

Demethylation Agents

Azacitidine and Decitabine work through demethylation. In normal cellular metabolism, cytosine methylation is an epigenetic mark for maintenance of gene silencing across cellular divisions. However, this chemically stable modification may be removed from DNA through demethylation. Azacitidine and Decitabine are chemical analogs of cytidine. Like other antimetabolites they can become incorporated into the nucleic acid.